Collection: Battery charge relay

Battery charging relays: distributing charge without mixing battery banks

An automatic charging relay, often called an ACR (Automatic Charging Relay), connects two battery banks only when a charging source is actually available. While sailing, it allows, for example, charging the house bank from the engine alternator while keeping the start battery isolated when the voltage drops.

This collection includes Blue Sea Systems charging relays suitable for 12 V and 24 V marine installations: compact 65 A models, 120 A relays with or without start isolation, 500 A ML-ACR magnetic latch relays, and Add-A-Battery kits combining ACR and Dual Circuit Plus battery switch.

Choosing the right rating: 65 A, 120 A or 500 A

• Blue Sea m-Series 7601 - 65 A: compact ACR relay for small 12/24 V installations, moderate alternators, powered dinghies, lightweight motorboats, or simple refits with two batteries.
• BatteryLink 7611 - 120 A: 12/24 V solution for two battery banks, with 210 A intermittent rating, when a simple automatic coupling is needed.
• SI-ACR 7610 - 120 A: 12/24 V model with Start Isolation, useful for protecting electronics, chartplotters, and computers from voltage drops or spikes during engine starts.
• ML-ACR 7620 - 12 V 500 A, ML-ACR 7622B - 12 V 500 A with manual control and ML-ACR 7623 - 24 V 500 A: magnetic latch relays for high-output alternators, large battery banks, and high starting currents.
• Mini Add-A-Battery 7649 - 65 A and Add-A-Battery 7650 - 120 A: complete kits with ACR relay and Dual Circuit Plus battery switch, useful when a clean start + house bank system must be created.

Technical points to check before ordering

• Nominal voltage: most ACRs in this selection accept 12/24 V, but ML-ACR models are available in dedicated 12 V or 24 V versions.
• Continuous current: 65 A for small systems, 120 A for common setups, 500 A for high-output alternators, large banks, or battery combining needs.
• Coupling and opening thresholds: Blue Sea models typically combine around 13.0–13.6 V in 12 V systems, then isolate the bank when voltage drops to around 12.35–12.75 V. In 24 V systems, thresholds are doubled.
• Start Isolation: recommended if the onboard electronics network resets or drops out during engine starts.
• Marine protection: models in this range are IP66 or IP67 depending on the reference, with ignition protection for gasoline engine compartments on relevant references.
• Wiring: a 500 A relay only makes sense if cable gauge, terminals, fuses, and battery switches are sized accordingly.

ACR, diode isolator, or DC/DC charger?

An ACR relay is effective when sharing a charging source between two banks with the same electrical logic, with minimal losses and no diode voltage drop. It is not automatically the best solution for all lithium architectures: whenever the house bank is LiFePO4, verify the BMS, alternator protection, charge curve, and current limits. In some cases, a DC/DC charger or a BMS-controlled architecture will be cleaner than a simple coupling relay.

Skysat advice

Do not choose a charging relay based solely on price or maximum rating. Start with the alternator, number of battery banks, battery chemistry, cable length, and expected behavior during engine starts. For a classic engine + house bank setup in lead-acid, AGM, or gel, an SI-ACR 120 A or an Add-A-Battery kit often covers the need. For large banks, high-output alternators, or manual combining needs, the ML-ACR 500 A family is more appropriate.

Pair with battery switches, fuses, busbars, and battery monitors to create a clean, maintainable architecture.